People can exercise only so much self-control

By Sophie Terbush, USA TODAY

People who overtax their self-control may find they have less in reserve for later, suggests an intriguing new study that may have implications for people trying to lose weight or make other behavioral changes.

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But lack of sleep does not appear to affect self-control, say the researchers, whose study of 58 subjects is in the March issue of the journal Social Psychological and Personality Science.

The subjects — half had stayed awake for 24 hours and half were well-rested — were shown scenes involving vomit and excrement from two movies, Monty Python's The Meaning of Life (1983) and Trainspotting (1996).

Some were allowed to express reactions; others were told to show no emotion. Later, they played an aggressive game in which they won or lost by chance. Winners were allowed to blast their opponent with a loud noise.

Those who had suppressed their emotions blasted their opponent at a noise level about 33% higher than those who were allowed to show emotion, regardless of how much sleep they'd had, researchers found.

Results suggest that "people have a diminishable supply of energy that the body and mind use to engage in self-control," says study author Kathleen Vohs, a consumer psychology professor at the University of Minnesota's Carlson School of Management. "When people use this energy toward achieving one goal, they have less of it available to use toward achieving other goals."

That can help predict when people are likely to fail at their diets, spend too much money or misbehave with family or in relationships, Vohs says.

Results suggest loss of self-control resources isn't the same as being tired, she says. "The ability to engage in self-control is determined by prior use of self-control, not by how much sleep one had the night before."

The study was part of ongoing research on sleep deprivation at the University of Texas-Austin.

Findings don't suggest busy people will lash out for no reason: "Aggressive behavior involves some action by someone else that causes you to want to retaliate," says researcher Art Markman, a psychology professor at the University of Texas.

Roy Baumeister, director of social psychology at Florida State University, has done extensive research on self-control. "Most people chronically don't get enough sleep, so it's reassuring to suggest from this one finding that it does not have any effect on self-control of aggression," he says.

But Baumeister says the test used may not account for other factors besides self-control that could contribute to aggression, such as personality or the competitiveness of the task itself.

Sian Beilock, a psychology professor at the University of Chicago, says it's interesting that "being taxed in terms of doing one task can have these spillover effects on another." People may think they can compartmentalize the different tasks they do during the day, but it turns out they are all connected, she says.

The study, paid for in part by the U.S. Army, could have important implications for the military as well. Though a lab is nothing like a war zone, "it does give preliminary reason for hope that just because a soldier has been forced to stay up for 24-36 hours, it doesn't mean they will react aggressively because they were sleep-deprived," Markman says.

For the rest of us, Vohs recommends being more mindful of priorities:

"When you want to engage in good self-control, the best thing that you can do for yourself is set up your day so you exert your self-control resources toward that specific task you want to succeed at."

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